The present invention relates to the shape of electrodes constituting the main lens of the electron gun of a color picture tube and to voltage application to each of the electrodes.
FIG. 1 is a plan view of a color picture tube provided with an electron gun having the conventional structure. A phosphor screen 3 on which stripes of phosphors in three colors are alternately coated is supported on the inner wall of a face plate 2 of a glass vacuum envelope 1. Central axes 16, 17, and 18 of cathodes 6, 7, and 8 coincide with the central axes of the apertures of a G1 electrode 9, a G2 electrode 10, a focussing electrode unit or assembly 12 comprised of one or more members and hereafter referred to as a focusing electrode constituting a main lens and a shield cup 14 which correspond to the respective cathodes and are arranged almost in parallel with each other on the common plane. Although the central axis of the center aperture of an accelerating electrode 13 which is another electrode constituting the main lens coincides with the aforementioned central axis 17, central axes 19 and 20 of the side apertures do not coincide with the central axes 16 and 18 which correspond to them respectively and are slightly displaced outside. Three electron beams emanated from each cathode enter the main lens along the central axes 16, 17, and 18, respectively. A focussing voltage of about 5 to 10 kV is applied to the focussing electrode 12 and an accelerating voltage of about 20 to 30 kV is applied to the accelerating electrode 13 so as to provide the same potentials as those of the shield cup 14 and a conductive coating 5 installed inside the glass vacuum envelope. The center apertures of the focussing and accelerating electrodes are coaxial with each other, so that the main lens which is formed at the center is rotationally symmetrical and the center beam is focussed by the main lens and goes straight on the path along the axis. On the other hand, the central axes of the side apertures of both the electrodes are displaced from each other, so that a rotationally asymmetrical main lenses are formed on both sides. As a result, side beams pass through the part dislocated from the central axis of the lens toward the center beam in the diverging lens area formed on the accelerating electrode side in the main lens area and are applied with the converging force toward the central beam as well as focussing action by the main lens. In this way, the three electron beams converge so as to overlap each other at an aperture of a shadow mask 4 as well as focus. An operation for converging three beams in this way is called static convergence (hereinafter abbreviated to STC). Furthermore, each electron beam is subjected to color selection by the shadow mask and only a portion of each beam which excites the phosphor of the intended color corresponding to each beam so as to emit light passes through the aperture of the shadow mask and reaches the phosphor screen. To allow the electron beams to scan on the phosphor screen, a magnetic deflection yoke 15 external to a color picture tube is installed around the neck portion of the vacuum envelope 1.
It is known that by combining an in-line electron gun in which three initial electron beam paths are arranged on a horizontal plane as mentioned above and a so-called self-convergent deflection yoke for forming a special nonuniform magnetic field distribution, if the three electron beams are statically converged at the center of the screen, they can be converged over the entire screen. However, when the self-convergent deflection yoke is used, the deflection aberration is increased due to nonuniformity of the magnetic field distribution and the resolution in the peripheral area of the screen is reduced. FIG. 2 shows beam spots on the screen distorted due to deflection aberration schematically. In the peripheral area of the screen, a high brightness portion c (core) of the electron beam spot which is indicated by diagonal lines extends horizontally and a low brightness portion h (halo) extends vertically.
A means for solving this problem is indicated in Japanese Patent Application Laid-Open No. 2-72546. FIG. 3 shows an example of the structure of a conventional electron gun. The focussing electrode 112 is divided into two parts in the direction from the cathode to the phosphor screen, such as a first member 127 and a second member 128. In the end face of the second member 128 which is opposite to the first member 127, flat electrodes 124 are installed above and under the electron beam passing aperture and extended into the first member via the single opening installed in the end face of the first member which is opposite to the second member. Inside the first member 127, an electrode 125 with an electron beam passing aperture provided is arranged at a fixed interval from the flat electrodes 124. A voltage which varies dynamically in synchronization with the deflection current supplied to the deflection yoke, that is, a dynamic focus voltage Vd is given to the second member 128 and the flat electrodes 124 together with a focussing voltage Vf superposed. When the amount of deflection is large, the potential difference between the first and second members is increased, so that the quadrupole lens effect of a rotationally asymmetrical electron lens formed by the flat electrodes is increased and a great astigmatic aberration is generated in the electron beam passing between the aforementioned flat electrodes. When the potential of the second member 128 is higher than that of the first member 127, an astigmatic aberration generated in the electron beam has an effect for extending the core vertically and the halo horizontally. Therefore, the astigmatic aberration accompanying the electron beam deflection shown in FIG. 2 can be offset and the resolution in the peripheral area of the screen can be improved. On the other hand, when the electron beam is not deflected, by eliminating the potential difference between the first and second members, no rotationally asymmetrical electron lens is formed and astigmatic aberration can be eliminated at the center of the screen. Therefore, the resolution will not be degraded.
In the color picture tube, the distance from the main lens to the peripheral area of the screen is longer than the distance from the main lens to the center of the screen. Therefore, the voltage condition for focussing the electron beam is different between the center and peripheral area of the screen. Under the voltage condition for focussing the electron beam at the center of the screen, the electron beam in the peripheral area is not focussed and the resolution becomes worse. This is referred to as curvature-of-field aberration. However, in a conventional example shown in FIG. 3, when the electron beam is deflected to the peripheral area of the screen, the potential of the second member 128 is increased, so that the voltage difference from the accelerating voltage of the accelerating electrode 13 is reduced and the lens strength of the main lens is decreased. Therefore, the focus point of an electron beam is moved toward the phosphor screen and the electron beam can be focussed on the phosphor screen even in the peripheral area of the screen. As a result, the resolution in the peripheral area can be prevented from degradation. Namely, a dynamic correction of astigmatic aberration as well as a dynamic correction of curvature-of-field aberration can be realized.
However, in a cathode ray tube of wide angle deflection, the deflection aberration is increased, so that a dynamic focus voltage which is a comparatively high voltage of more than 1 kV is necessary so as to correct it.
According to the aforementioned prior art, a cathode ray tube of wide angle deflection requires a dynamic focus voltage which is a comparatively high voltage and for that purpose, the cost of a dynamic focus voltage generating circuit is increased inevitably due to its high voltage or the deflection aberration is not corrected fully due to an insufficient amplitude of the dynamic focus voltage and the resolution in the peripheral area is degraded.